docs: def path(self, filename):
          '''
          This returns the absolute path of a file uploaded to this set. It
          doesn't actually check whether said file exists.
  
          :param filename: The filename to return the path for.
          :param folder: The subfolder within the upload set previously used
                         to save to.
  
          :raises OperationNotSupported: when the backenddoesn't support direct file access
          '''
          if not self.backend.root:
              raise OperationNotSupported(
                  'Direct file access is not supported by ' +
                  self.backend.__class__.__name__
              )
          return os.path.join(self.backend.root, filename)